offgassing Ni or Cr from heating stainless steel 1

[bobbbb]

30 Dec 11 19:41
Hi all, New to forum.
Not an exhaustive search but enough to frustrate me. Cant seem to find any readily available solid information out there regarding this issue. I'm more of a fabricator and cant present myself as a bona fide engineer, so I'm hoping there is a metallurgist on this forum that can shed a little light on the subject. I am constructing a heat exchanger(using air flow to transfer heat, not liquid) where the heat source temperature is somewhat uncontrollable, like in a naturally aspirated wood burning stove, and the system will use natural convection flow to create a draft as the primary mover of exchanged air and not a fan forced air that would naturally provide additional heat exchanger material cooling.
The last unit I built was just carbon steel. but since the heat exchanger is buried in the coal bed I have long term service life concerns where heat scale or corrosion from caustic ash may eat away enough material to cause air leaks similar to what is seen in an automotive tailpipe after 5 seasons of road salt, in turn contaminating air with smoke, CO, etc. that will be circulated into an inhabited room rendering the unit unsuitable for indoor use. So I would like to explore stainless as a more corrosion/scale resistant material option for a longer service life.
My concern is any off-gassing of toxic heavy metals like Ni or Cr while in service(as I am cautioned against while welding these materials), and/or material scaling or thermal fatigue from high heat and potentially daily heat/cool cycles ultimately leading to cracking as I have experienced in some alternative fuel experiments where I used a cheap stainless steel salad bowl as a flame retainer that eventually crumbled after repeated heating and cooling.
So I guess the big question would be are these valid concerns? At what temperature or material state does stainless start off-gassing any toxic/heavy metal fumes or vapors? A little off point, but can a wood fire, either naturally or fan aspirated create enough heat to cause scaling and this off-gassing condition? I have read 3600*F is adiabatic temp. of wood(after further exploring a response from someone on a different section of this site regarding the same basic questions) so would probably be common in the coal bed of a hot wood stove. Would a specific alloy make a difference in this application? I know 321 is typically used in high heat applications where pressure strength needs to be retained like jet engines, but is also costly for this. Will thermal expansion cracking or heat scaling be an issue with an alloy that is readily available like 303 or 304? What would be the most ideal material from a suitability/cost effective relationship for an application like this?
Thanks in advance for any solid light anyone can shed on this issue or point me to other resources/forums discussing this issue, and feel free to make it a discussion thread as I'm sure I'm not the only one pondering this question for one reason or another. P.S.: I'm ok with logical speculations as they sometimes lead to great ideas, useful debate or point one in the right direction even if it is inaccurate upfront. However, if you don't actually know and are making a logical speculation that can't be backed up by an industry reference please state it as such so no one takes it as gospel.
Thanks again and happy new year everyone.

 

[metengr]

bobbbb;
Typically, concerns related to hexavalent chromium vapors are valid when the chromium-containing steel is in a molten state, like during welding. If your service conditions result in exposure of the stainless steel to temperatures in excess of 1400 deg F but below the melting point, you can locally carburize the surface of the stainless steel in a reducing environment enriched with carbon. Corrosion resistance will be affected, however, this should not result in generation of harmful vapors.

 

[EdStainless]

There will be no evolution of Cr or Ni under your conditions. With oxygen present (air) you have to get above melting to volatilize any metals since they would rather oxidize and the oxides will remain solid.

Alloy selection is another matter. You could use 321H, it does not cost much more than 304. It will maintain properties better at high temp, but it has the same (very large) thermal expansion as 304.
A better option would be a ferritic stainless. 430 might be you best bet. This alloy is basically just Fe and Cr. It has good oxidation resistance, good heat transfer, and lower thermal expansion.
Remember to keep your parts as thin as possible. The variations in temp and heating and cooling will cause distortion. If you make things strong enough to resist the distortion then they will tear themselves apart. The stress has to go somewhere.

= = = = = = = = = = = = = = = = = = = =
Plymouth Tube

 

[btrueblood]

A friend built his wood stove from Inconel 625 (1/16" thick or so). Lasted for years. We would fire it to red heat, and never saw any scaling.

 

[bobbbb]

Thats awesome, thanks for the input guys. Met and Ed you're solidifying what a couple others said, regarding the material actually having to melt in order to thermally mobilize heavy metals in fumes or vapors. Now that more and more health concerns surrounding heavy metals like mercury, Aluminum, hex Chrome and the use of certain materials in cooking and food storage containers are being raised, it also relieves questions I have had about the ni-chrome wire they use in space heaters, toasters and other appliance that utilize glowing elements with direct air exposure to breathing air or food, there's just little to no real information readily available on this subject without extensive educative knowledge or research on the subject.
And keeping materials on the the thinner side makes sense Ed. Not only would it to give it freedom to expand and contract without mechanically binding itself, heat transfer would be faster making the exchange more efficient. Also, interesting thoughts on using 430, I have always associated the use of Ferritic alloys in applications where where heat treating is required for hardness, toughness or both but not having to completely abandon corrosion resistance.
The last unit I fabricated to fit a fireplace was to help my brother in law out when they killed his utilities a couple years ago after the bottom fell out of the local economy. My basic design utilized tubing due to it's simplicity of cutting, fitting and welding. I made that one of carbon steel and was all miscellaneous scrap tubing I had sitting around. Most was more or less .062" and the exchanged air drafts out 250-300*F with a roaring fire.
Ed, Mechanically speaking for integrity protection from thermal expansion would thinner walls be more of an advantage on round or rectangle tubing? Part of my design is utilizing the exchanger as a structure to support the fuel source. That said, which would be less susceptible to self destruction from thermal stress or is the difference negligible and I'm chasing this rabbit too far down the hole? Logic says the rectangle tubing is more rigid as a support and provides more exchangeable surface area per air volume unit than round tubing, but also has sharply bent edges with more rigid points of stress, where round tubing would seem to have a more uniform surface with the constant curve to expand and contract like a balloon without putting additional stress on rest of the system, being void of sharply bent edges. The tubes that see the most heat on the last unit had a 1"x3" cross section if my memory serves me correctly, with somewhere in the neighborhood of .063" wall(maybe a gage under but probably not over, as that gets noticeably heavier feel to it). I welded them far enough apart to hold the fire together with enough gap for ash to fall through. This next unit I want to space the tubes far enough apart to allow the coals to be able to fall down through the gaps as they crumble so as to immerse the exchanger in the coal bed but not far enough to over-cool the fire. That said, assuming I use 321, 430 or something very similar; Any recommendations on an optimal material thickness that will be thin enough to move, but still thick enough to maintain it's structural integrity so it doesn't collapse on itself under the weight of a few good logs if it really gets hot?
Thanks also btrue, I searched the properties of Inconel 625, from what I found thus far, it was actually developed for application of this type, that would experience extreme conditions, and worth looking into a little more.
Thanks again all and I certainly welcome more input and experiences along the same lines on these matters, as knowledge is one of those commodities that there is always "space for more" of, and one never knows when they may use a piece of knowledge tomorrow that was randomly obtained yesterday.

 

[btrueblood]

FWIW, the stove in question needed to be light, so it could be packed into the mountains ~12 miles on horseback, and used in elk hunting camp. It took a lot of abuse.

Thin sections of any shape will transfer heat better than thick sections, but - the material must retain strength at whatever temperature it's going to run at, and any loss of material due to corrosion/oxidation will reduce life much more quickly, which tends to push back on low-chrome alloys.